Infrared Spectroscopy of Carboions. 8. Hollow Cathode Spectroscopy of Protonated Acetylene, C2H3+

Abstract

The vibration-rotation energy level pattem of protonated acetylene, C2H3+, in the v6 (C-H antisymmetric stretching) vibrational state is anomalous and irregular because of the coupling between the rotational motion and the tunneling of the three protons among their equilibrium positions. The resultant spectral anomaly and the coexistence of C-H bands of other carbocations such as CH3+, C2H2+, CH2+, etc., in our positive column discharge using He-dominated gas mixtures had made the analysis of the C2H3+ spectrum difficult. In the present paper we use a hollow cathode discharge to simplify plasma chemistry and to make a more definitive and extensive analysis of the spectrum. A 3 m hollow cathode discharge cell has been constructed with a multiple reflection optical system giving an effective path length of 30 m. A gas mixture of C2H2 and H2 with pressures of 0.03 and 1.1 Torr, respectively, has produced spectral lines of C2H3' from 3192 to 3083 cm-' which are almost completely free of those from other carbocations. The purity of the spectrum, together with the accurate ground state rotational constants recently reported by the Lille millimeter wave group, has allowed us to assign spectral lines up to J = 25 and K, = 4 and to determine extensive sets of the A-E splittings due to the proton tunneling in the excited state. An attempt has been made to analyze the plasma chemistry in the hollow cathode on the basis of earlier plasma diagnostic studies of the negative glow region. A model was used in which the primary molecular ions H2+ and C2H2+ are generated due to ionization by "hot" primary and secondary electrons. In the plasma these ions undergo ion-neutral reactions to produce H3+ and C2H3+, which are dissociated by reactions with C2H2 and recombination with "ultimate" electrons. By assuming proper number densities of primary, secondary, and ultimate electrons, semiquantitative agreement with the experimentally estimated ion densities has been obtained.

Primary photodissociation pathways of epichlorohydrin and analysis of the C–C bond fission channels from an O(P3)+allyl radical intermediate

“…The P͑E T ͒ in the middle frame of Fig. 50,51 Thus, we presume that the apparent earlier onset in the data of Neumark and co-workers is due to hot bands in the photoionization spectrum. 21, bottom.…”

Quantum astrochemical spectroscopy

“…The CcCR QFF computations actually put the antisymmetric CAH stretch for the lowest-energy, cyclic isomer within 0.1 cm 21 of the experimental frequency at 3142.2 cm 21 , the B and C rotational constants within 4.0 MHz at 34237.5 and 31371.8 MHz, and the D J value within 0.7 kHz of experiment at 37.65 MHz. [69,79,80] The CcCR QFF is still a very trustworthy tool. The cyclic form of C 3 H 2 has also been suggested as a potential interstellar species, as well, based on CcCR QFF computations, [81] but l-C 3 H 1 is present in the Horsehead nebula PDR.…”

Unimolecular dynamics from kinetic energy release distributions. V. How does the efficiency of phase space sampling vary with internal energy?

“…Numerous reliable ab initio calculations 36,37 as well as a detailed spectroscopic analysis 38,39 have established that the minimum energy structure of the C 2 H 3 ϩ ion corresponds to a nonclassical bridged structure belonging to the C 2v point group, which can be described as protonated acetylene, hereafter denoted structure 1. The classical, Y-shaped, vinyl structure H 2 CCH ϩ ͑denoted 2͒ is found to lie about 0.13 eV higher in energy and is now believed to be very close to a transition state ͑denoted TS 12 ͒ in an internal rotation motion during which the three hydrogen atoms rotate as an equilateral triangle about a dumbell of two carbon atoms ͑Fig.…”